Howdyo,
The multiple session support should be usable now. Each connection gets its own REPL, and if you do `C-u M-x slime' you get the option of firing up an extra *inferior-lisp* to connect to. Note that currently if you bring up two inferior-lisps, the first one will be called *inferior-lisp*<1> and the second *inferior-lisp*.
As before, `slime-next-connection' will cycle the "default" connection that will be used for commands in your lisp-mode buffers. The REPL, SLDB, apropos-results, etc buffers are all "sticky" -- they keep using the connection that created them. Currently there isn't much to help you keep track of matters, except that repl/sldb buffers have the connection number in their buffer name.
Obligatory screenshot (slightly gimp-mangled, I'm no Dan Barlow :-) http://www.bluetail.com/~luke/misc/lisp/slime-multisession.png
Bug reports or usability suggestions would be welcome.
Cheers, Luke